Co-founder Biz Stone said the micro-blogging service still had money in the bank from two earlier funding rounds, which totaled $20 million, but Twitter received "an offer we couldn't refuse."

"Our strong growth attracted interest, and we decided to accept a unique opportunity to make Twitter even stronger with a very attractive offer," he wrote in a corporate blog post titled "Opportunity Knocks."

Stone said Twitter's active user base had grown 900% in the last year -- though he didn't mention the total. The company's Web traffic was "amazing" too, he said, but it was far outpaced by the traffic through the tools Twitter makes available to outside programmers.

The service's growth has been powered by a thriving ecosystem of developers who create programs that make Twitter more useful.